This invention relates to a field mounted measurement transmitter measuring a process variable representative of a process, and more particularly, to such transmitters which have a microprocessor.
Measurement transmitters sensing two process variables, such as differential pressure and a line pressure of a fluid flowing in a pipe, are known. The transmitters typically are mounted in the field of a process control industry installation where power consumption is a concern. Measurement transmitters provide a current output representative of the variable they are sensing, where the magnitude of current varies between 4-20 mA as a function of the sensed process variable. The current needed to operate a measurement transmitter must be less than 4 mA in order for the transmitter to adhere to this process control industry communications standard. Other measurement transmitters sense process grade temperature of the fluid. Each of the transmitters requires a costly and potentially unsafe intrusion into the pipe, and each of the transmitters consumes a maximum of 20 mA of current at 12V.
Gas flow computers sometimes include pressure sensing means common to a pressure sensing measurement transmitter. Existing gas flow computers are mounted in process control industry plants for precise process control, in custody transfer applications to monitor the quantity of hydrocarbons transferred and sometimes at well heads to monitor the natural gas or hydrocarbon output of the well. Such flow computers provide an output representative of mass flow rate as a function of three process variables. The three process variables are the differential pressure across an orifice plate in the pipe conducting the flow, the line pressure of the fluid in the pipe and the process temperature of the fluid. Many flow computers receive the three required process variables from separate transmitters, and therefore include only computational capabilities. One existing flow computer has two housings: a first housing which includes differential and line pressure sensors and a second transmitter-like housing which receives an RTD input representative of the fluid temperature. The temperature measurement is signal conditioned in the second housing and transmitted to the first housing where the gas flow is computed.
Methods of measuring natural gas flow are specified in Orifice Metering of Natural Gas and other Related Hydrocarbon Fluids, Parts 1-4, which is commonly known as AGA Report No. 3. Calculating the mass flow rate requires that the compressibility factor for the gas and the orifice discharge coefficient be computed. The compressibility factor is the subject of several standards mandating the manner in which the calculation is made. Computing the compressibility factor according to these standards expends many instruction cycles resulting in a significant amount of computing time for each calculation of mass flow and a large power expenditure. Accordingly, the amount of time between subsequent updates of the mass flow rate output is undesirably long if each update is calculated from a newly computed compressibility factor, so as to slow down a process control loop. Even if the compressibility factor is calculated in the background so as to prevent lengthening the update rate, the mass flow rate output is calculated from a stale compressibility factor which provides poor control when the process changes rapidly. Furthermore, calculation of the compressibility factor entails storage of large numbers of auxiliary constants which also consumes a large amount of power. AGA Report No. 3 Part 4 mandates mass flow rate accuracy of 0.005%, resulting either in slow update times, use of stale compressibility factors in computing mass flow rate or power consumption greater than 4 mA. Similarly, direct calculation of the orifice discharge coefficient requires raising many numbers to non-integer powers, which is computationally intensive for low power applications. This also results in undesirably long times between updates or power consumption greater than mandated by the 4-20 mA industry standard.
There is thus a need for a field mounted multivariable transmitter adaptable for use as a gas flow transmitter having improved update times, but consuming less than 4 mA at 12V of power without sacrificing the accuracy of the calculation.
Another aspect of the present invention relates to pressure measurement devices, and particularly to pressure transmitter systems that respond to pressure at least two discrete locations and that communicate with a separate controller over a two-wire link.
Pressure transmitters having a transmitter housing that includes a differential pressure (".DELTA.P") transducer fluidically coupled to two pressure ports in the housing, are known. Such transmitters further include in the transmitter housing circuitry coupled to the transducer and communicating the measured .DELTA.P to a distant controller over a two-wire link. The controller energizes the circuitry over the two-wire link. Fluid conduits such as pipes or manifolds carry a process fluid to the transmitter pressure ports. Typically, process fluid immediately upstream and downstream of an orifice plate is routed to the respective ports, such that the .DELTA.P measured by the transducer is indicative of process fluid flow rate through the orifice plate.
In some applications it is desired to measure differential process fluid pressure at locations separated from each other by a distance much greater than the scale size of the transmitter housing. To make such a measurement it is known to attach to the above described .DELTA.P transmitter flexible oil-filled capillary tubes or impulse piping to fluidically transmit the process fluid pressures to the housing pressure ports. However, such arrangements suffer from errors due to differences in height and temperature of the oil-filled capillary tubes.
It is also known to provide a separate pressure transmitter at each of the two process fluid measurement locations, and to electrically couple each of the pressure transmitters to a "hydrostatic interface unit" (HIU). The HIU communicates with the distant controller over a two-wire link, and is powered by a separate unit over a different electrical link. The HIU, in turn, electrically powers and communicates with the pressure transmitters, and performs multiple arithmetic operations on the measured pressures. For example, where the pressure transmitters are mounted on a storage tank of process fluid, the HIU can communicate over the two-wire link a 4-20 mA signal indicative of the process fluid density .rho.: ##EQU1## where .DELTA.P is the process fluid pressure difference between the transmitters, g is gravitational acceleration, and z is the (user-programmed) vertical separation of the fluid measurement locations. This system avoids problems associated with oil-filled capillaries external to the transmitter housing, but has disadvantages of its own such as the need to mount additional electronic devices proximate the measurement site and the need for a separate power supply for the HIU due in part to the large number of calculations performed by the HIU.